My understanding is that the sprag (MB calls it a freewheel-unit) is bad. Joe P did an excellent post about this problem.
Dealers insist "they all do that" and ignore you, like they do with every problem that doesn't bring the car in on a hook.
My 1998 C230 has a mild case of it, and the dealer replaced the shifter bushings. Huh? Of course, it remains unsolved. Otherwise, my tranny is great, but that "clunk" is annoying. BTW, I know it's not normal, as my Dad's 1998 C230 doesn't do it at all...
1998 C230 "Black Betty"